The interaction of rat macrophages with activated T cells is being studied in situ with a view to determining the specificity requirements necessary for the triggering of T cells and generation of mediators that have secondary effects on blood monocytes, monocyte-derived macrophages and circulating lymphocytes. Prominence is being given to the part played by products of the major histocompatibility complex (MHC) in the recognition of bacteral antigens. Preliminary evidence indiiates that while such products are important in the rats as they are in the mouse, they are not totally restrictive in the recognition of antigens expressed by the intracellular bacterial parasite, Listeria monocytogenes. Experiments addressed to this issue make use of a variety of labeling techniques and quantitative microbiological methods to assay the accumulation of Listeria-induced exudates of defined populations of lymphocytes and macrophages, the local generation of lymphokines and the capacity of the host to defend itself aginst a challenge infection with L. monocytogenes.